The present invention relates to a color picture tube in which a high resolution is sought for over the entire area of the phosphor screen.
The resolution of a color picture tube can be improved by reducing the spot diameter of an electron beam. It is known that this can be achieved by increasing the diameter of the main lens formed by focusing grids and a final accelerating grid.
The conventional in-line electron gun, as disclosed in U.S. Pat. No. 5,142,189, employs a method of increasing the diameter of the main lens by superposing the lens fields for focusing the three beams of R, G, B.
In the above-mentioned conventional method, however, the horizontal diameter of the main lens becomes smaller than the vertical diameter thereof. In the case where the horizontally elongated distortion becomes conspicuous with the sectional shape of the peripheral spot elongated in horizontal direction due to the trend toward the flattening of the phosphor screen panel and the increased deflection angle, the above-mentioned conventional method is disadvantageous for reducing the horizontally elongated distortion by reducing the horizontal diameter of a spot on the periphery of the color picture tube (hereinafter referred to as the peripheral spot).
It has been known that the diameter of the main lens can be increased by increasing the distances from the positions of the field-correcting electrodes arranged on the focusing grids and the final accelerating grid to the respective opening ends, and thus increasing the degree of superposition of the lens fields. This results in excessively small center distance between the center lens and the side lenses, however, making it necessary to increase the distance between the shadow mask and the phosphor. The color picture tube thus is easily affected by the geomagnetism. As a result, a color drift is liable to occur as the electron beam fails to impinge correctly on the phosphor. A solution to this problem has long been sought for.
In order to solve this problem, the object of the present invention is to provide a color picture tube in which the horizontal lens diameter is increased without much reducing the center distance between the center lens and the side lenses as compared with the prior art.